Foil sound absorbers

ABSTRACT

A foil sound absorber consists of at least two smooth, flat and air-tight foils separated from each other and from a reverberant rear wall R by different distances D.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a foil absorber for sound.

1. Introduction

The use of more or less homogeneous layers of fibrous/porous materials(e.g. synthetic mineral fibers (KMF)) predominate as seen in FIGS. 1A-C:in room acoustics (e.g. wall and ceiling cladding, FIG. 1a), in noisecontrol of loud machines (e.g. cladding and screening) as well as intechnological noise control (e.g. by means of sound dampers in flowcanals, FIG. 1c). The discussions for many years about possible healthrisks from the fine dust and fibers (Koster, J.; Grunau, E.B.:Mineralfasern: Eine Gefahrenquelle, Expert-Verlag, Ehningen, 1993) aswell as from deposits and germ infestation in such layers have led, onthe one hand, to attempts to cover and wrap the porous material withsuitable foils and nonwoven fabrics and, on the other hand, to seekalternative sound absorbers obviating the use of porous materials. Thissearch soon led the assignee of the present invention to the developmentof three totally different absorbers for completely different acousticalapplications ("Schallabsorbierendes Bauelement"--German patent documentDE 27 58 041; "Schalldampfer Box"--German patent document DE 35 04 208(corresponding to U.S. Pat. No. 4,787,473); and "schallabsorbierendesGlas--oder Kunstglas-Bauteil"--German patent document DE 43 15 759).These absorbers can, each individually and in combination, as well as asa supplement to enhance the performance of conventional absorbers, covera broad frequency range of approximately 50 Hz up into the kHz range anda very wide field of applications. Moreover, there are also more or lesssuccessful attempts at describing these usually two-dimensionallydesigned absorbers.

For instance, in Mechel F.; Kiesewetter, N.; Schallabsorber ausKunststoff-Folie. Acustica 47 (1981), pp. 83-88 shows that a planeplastic foil that is excited to propagate forced bending waves by adiagonally striking soundwave is unable to destroy a substantial part ofthe sound energy by means of internal friction. In order to,nonetheless, substantially raise the degree of dissipation, it has beenproposed to deform the foil in such a manner that rectangulartwo-dimensional pieces of a few centimeters in length and width, whichare bordered by a fold, are created. The fold at the edge of theseplates acts like a fastening device and prevents free motion of the foilat this site. By this means, these plates are excited to characteristicvibrations. The wavelengths of these natural vibrations are in thefrequency range up to 5000 Hz substantially smaller than the trackwavelength of the incident airborne soundwave. The vibration amplitudeof the plate is especially great in the case of natural frequencies. Dueto the resonance-like co-vibration of the subareas with theircharacteristic bending vibrations, one thought if there continues to berelatively little internal dampening, but stronger deformation of thefoil material, greater dampening of the excited soundwaves could beachieved at least in the vicinity of the natural frequency. In order toobtain a wideband sound absorber despite these profiled foils which areprincipally limited only functioning in narrow frequency bands, onesolution described in "Schallabsorbierendes Baulelement" German patentdocument DE 29 21 050 (corresponding to U.S. Pat. No. 4,425,981), and"Schallabsorbierendes Bauelement"--German patent document DE 32 33 654(corresponding to U.S. Pat. No. 4,555,433) tried:

(1) to create plates of varying size swedging in the bottom and lateralsurfaces of the foils;

(2) to make a greater number of natural frequencies excitable within oneand the same plate by preferring oblong instead of square subareas; and

(3) to make such a multiplicity of natural vibrations possible bymultiple, small and large deformations plus additional mass inclusionsinto the swedged foils so that practically all the interesting frequencyportions of the to be dampened sound field can be absorbed as fully aspossible.

This optimizing of ten or more simultaneously excitable bendingvibrations yields a greatly cleaved, rough sound absorber surface.

2. Disadvantage of the conventional foil absorbers

The above-described cupping, molding, structuring, swedging, creasingand grooving in the hitherto employed foil absorbers have a number ofmajor drawbacks: they can hitherto only be successfully made atreasonable cost with certain polyvinylchloride-based (PVC) foils. Otherplastic foils having comparable internal losses for energy dissipationof the bending vibrations do not permit these kinds of deformations.

PVC has in common with many other plastics that, even when used indoors,it is not permanently UV resistant and therefore may discolor. Forenvironmental concerns, the market is hesitant with regard to the use ofall PVC products. In some countries, fire prevention regulationsprohibit the use of large amounts of PVC in buildings.

As long as thin (0.2-0.4 mm thick) foils do not tear in swedging and arenot damaged during mounting as well as during maintenance, the enclosedhollow spaces which are formed permanently protect against penetrationof moisture and soiling. However, the characteristic, cleaved surfacenonetheless offers opportunities for deposits and soiling of all kindsin dusty, moist environments. In wet rooms, this can be counteracted tosome degree by washing and brushing the grime off. However, moreintensive and frequent cleaning has a negative effect on the durabilityof this type of foil absorber.

All these drawbacks considerably limit material selection for thefabrication of conventional foil absorbers as well as their applicationin the field of acoustics and the technological sound control for wallcladding and sound containment.

The object of the present invention is, therefore, to create a foilabsorber that is simple to fabricate and simple to clean.

According to the present invention, these objects are achieved by a foilsound absorber having at least two smooth, plane air impermeable foilshaving a surface weight M of approximately 0.05 to 1 kg/m². The foilsare disposed at a varying distance D from each other and from areverberant rear wall R. This distance D between the foils beingapproximately 5-100 mm.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C illustrate conventionally designed absorbers fortechnological sound control;

FIG. 2 is a schematic diagram of a foil absorber according to thepresent invention;

FIGS. 3A-D are graphical schematic illustrations of a simple resonancesystem built using only a single foil;

FIGS. 4A and 4B are a graphical and schematic illustration respectively,of a multi-layer foil absorber according to the present invention;

FIGS. 5A and 5B are a graphical and schematic illustration,respectively, of a foil absorber having three equally heavy foils;

FIGS. 6A and 6B are a graphical and schematic representation,respectively, comparing computed and measured values in a so-calledimpedence pipe;

FIGS. 7A-7C are graphical and schematic illustrations of a comparison ofmeasurements with computer results in a so-called echo chamber;

FIGS. 8A -8C are graphical and schematic illustrations of a comparisonof measurements with computer results in a so-called chamber;

FIGS. 9A and 9B are graphical and schematic illustrations, respectively,of a comparison of two-three sheet designed foil absorbers; and

FIGS. 10A and 10B are diagrams illustrating a plane foil absorber havingcoffered intermediate air spaces.

DETAILED DESCRIPTION OF THE DRAWINGS

The invented foil absorber completely prevents the cupshaped formationsand is comprised only of several (preferably 3) totally plane foilswhich are disposed in front of a reverberant rear wall (e.g. a veryheavy component) in series (preferably all in parallel to each other andto the wall). The foils can be made of any material, e.g. plastic ormetal. Its cross dimensions are largely freely selectable, e.g.corresponding to the respective case of the building which it is in.Their acoustic properties are determined according to FIG. 2 essentiallyby their surface weights m" and distances D to each other and to thewall. Contrary to conventional foil absorbers, the shape and formationof the hollow spaces between the foils and the wall, including themanner in which the foils are attached to spacers or to frames forattaching the absorber to the rear wall play, at least when the soundincidence is perpendicular, only a subordinate role. As the inventedabsorber does not essentially derive its action from the internaldampening in the foil material due to the excitation of bendingdeformations at folds, edges and supports, and hardly from frictionbetween the contact surfaces or friction of vibrating airborne particlesat the fine fibers or in narrow pores, regarding the selection ofmaterial and formation, it permits an adaption of the acoustic layoutfor the respective individual case that was previously not possible. Forthe layout on the same sound spectrum, it requires about the same depthand about the same surface weight as the known foil absorbers.

The invented foil absorber according to FIG. 2 is, similar to thoseaccording to German patent documents DE 27 58 041, DE 29 21 050 or DE 3233 654, a complex resonance system. With its small characteristicimpedance (cf. Fuchs, H.V.; Ackermann, U; Frommhold, W.: Entwicklung vonnicht porosen Absorbern fur den technischen Schallschutz. Bauphysik 11(1989), pp. 28-36), similar to the conventional foil absorbers, italready permits with a relatively small number of resonance mechanisms(preferably 3) an unanticipated wideband efficiency.

FIGS. 3A-D show an important optimizing principle of the inventedabsorber using the simplest example of a resonance system built of onlya single foil. For a wideband dampening of high frequencies, the foilshould have a small surface weight m" and not (in the case of largerm"), preferably, a correspondingly smaller distance. In order on theother hand to optimally absorb at low frequencies, not only should thesurface weight be increased but also simultaneously the thickness D ofthe air cushion. In this way, it is possible for even the simplestone-sheet design for deep frequencies to only require distinctly lessstructural depth than a homogeneously designed porous or fiber absorber.

This tendency is intensified even more so in multi-layer designedinvented foil absorbers. By attaching the foil 1, in addition in frontof the foil 2, in FIG. 4B, the part of the dampening curves (FIG. 4A)dropping to low frequencies shifts by one to two thirds.

FIG. 5A shows a computer generated result for three equally heavy foils(FIG. 5B) having an overall structural depth of 100 mm. The comparisonwith measurements in a so-called impedance pipe having a cross sectionof 200×200 mm² shows very good coincidence (FIG. 6A) up to a measurementlimit of 1200 Hz.

Measurements in the so-called echo chamber also follow the computerresults quite well, as shown for example in FIGS. 7A and 8A.

In FIG. 8A, in addition thereto, the measurement results of FIG. 2 fromGerman patent document DE 27 58 041 for a foil absorber of approximatelycomparable structural depth and surface weight were plotted. Thecomparison of two three-sheet designed foil absorbers accordingapproximately to FIG. 3 with A=50 mm is shown in FIG. 9A. Apparently,the deeper frequencies can be better absorbed with the invented foilabsorber, whereas the higher ones can be better absorbed with the foilabsorber according to German patent document DE 27 58 041.

The deficiency of the plane foil absorbers regarding oblique soundincidence generally predominating in echo chambers as well as in largerooms can be compensated by means of simple coffering of the largesurfaced air cushions. Moreover, it is necessary that the intermediateair spaces according to FIGS. 10A and 10B are divided by a uniform (FIG.10A) (e.g. honeycomblike) or non-uniform (FIG. 10B) (e.g. made ofcrunched foil) grid structure in such a manner that subareas the size ofa few centimeters are created. The coffering can occur by means ofintermediate walls made of plastic or metal. However, the foils shouldnot touch or lie on the internal coffers. The coffering can be suspendedor attached to the side borders of the foil absorber itself. Such anoptimization at high frequencies is known from, e.g. glancing incidentsound in sound damper valves in flow canals, which are thereforedesigned to be "coffered" on the interior. It is pointed out thatfrequently sound absorbers are called for in order to minimize impairingreflections in acoustics, which absorbers in particular can absorb theapproximately perpendicular striking sound waves.

The invented foil absorbers can be made to be practically randomlywideband absorbing by means of mass/spring systems disposed in astaggered relationship in series and composed of thin foils havingintermediate air spaces, in particular, if one raises (in the directionof sound incidence) the weight surface m" of the foils and alsoincreases the distances D between the foils in the direction to thewall.

It is especially advantageous if the hollow space resonator formed bythe foil 1, the lateral frames 2 or spacers and the rear wall R isdesigned in a gastight manner.

If, for instance, glass clear foils of acrylic glass having a thicknessof approximately 0.1 to 0.5 mm are selected, a completely transparentabsorber can be designed, which, at least in the case of perpendicularsound incidence, can optimally absorb the entire frequency range that isimportant for understanding language. (For frequencies above 1-2 kHz,there is usually sufficient absorption in multi-purpose rooms forlanguage and music due to the interior decoration of the audience.Compared to the plastic component according to German patent document DE43 15 759 with its micro-perforated hole plate as the sound absorber,the invented foil absorber can be designed acoustically more widebandand significantly less expensively and, due to the closed surface, so asto be easier to maintain.

On the other hand, if mechanically and chemically heavyduty foils ofplastics, metal or composite materials are selected, a very robust andwideband absorber for technological sound control obviating sensitivefibrous or porous materials can be designed.

The flat, completely plane and smooth construction of the invented foilabsorber offers substantial advantages regarding deposits and cleaning.

Compared to membrane absorbers according to German patent document DE 3404 208 with its complicated substructure of hollow chambers that aresealed from each other, the plane foil absorber can be fabricatedsignificantly cheaper and less expensively.

As it is free of any specific grid as a subconstruction or frame, theinvented foil absorber, like sound dampeners made of homogeneous mineralwool, can be built as a sound absorbing component having the requiredrigidity with any prefabricated elements and fabricated in modulespreferably in conjunction with a reverberant rear wall, as well as inany dimensions.

Another embodiment, e.g., for indoor swimming pools, can receive a thin,water-impermeable cloth as the first foil facing the room. An especiallyrobust variant can use a new, extremely tear-resistant, thin syntheticfabric as the first foil.

The invented foil absorber offers a variety of coloring and surfacestructure hitherto unknown for sound absorbers, which is beneficial forits use in acoustics.

What is claimed is:
 1. A foil sound absorber, comprising:a reverberantrear wall; and at least two smooth, planar air impermeable foils havingrespective surface weights m" of approximately 0.05-1 kg/m², said foilsbeing disposed in series with said rear wall at a varying distance Dfrom each other as well as a varying distance from said reverberant rearwall R, said varying distance D between said at least two foils beingapproximately 5-100 mm.
 2. The foil sound absorber according to claim 1,wherein said surface weights of said at least two foils increase towardsaid reverberant rear wall, and wherein said at least two foils aredisposed with an approximately equally increasing distance toward thereverberant rear wall.
 3. The foil sound absorber according to claim 1,further comprising:one of spacers and frames, said one of said spacersand frames being made of at least one of a metal, plastic, and acomposite material; and wherein outer edges of said at least two foilsare attached to said one of spacers and frames.
 4. The foil soundabsorber according to claim 1, wherein at least one of said foils has asurface area of approximately 0.1 to 1 m² which is suspended in freemanner.
 5. The foil sound absorber according to claim 1, wherein said atleast two foils are made of one of plastic, acrylic glass, metal, andcomposite materials.
 6. The foil sound absorber according to claim 1,wherein intermediate air spaces between said at least two foils arecoffered in one of a uniform and non-uniform manner, internal ones ofsaid coffers not hindering any of said at least two foils whenvibrating; andwherein said coffering prevents sound propagation in saidintermediate air spaces; wherein walls of said internal ones of saidcoffers are designed in a rigid manner and of the same materials.
 7. Thefoil sound absorber according to claim 3, wherein intermediate airspaces between said at least two foils are coffered in one of a uniformand non-uniform manner, internal ones of said coffers not hindering anyof said at least two foils when vibrating; andwherein said cofferingprevents sound propagation in said intermediate air spaces; whereinwalls of said internal ones of said coffers are designed in a rigidmanner and of different materials.
 8. The foil sound absorber accordingto claim 1, wherein one of said at least two foils remote from said rearwall is made of one of a water-impermeable fabric, cloth and syntheticfabric.
 9. The foil sound absorber according to claim 8, wherein saidone foil is at least one of dyed and printed.
 10. The foil soundabsorber according to claim 3, wherein a hollow space formed betweensaid foils, a frame and said rear wall is a gastight hollow space.